This invention relates to a process for the preparation of phosgene by reacting chlorine with carbon monoxide in the presence of activated charcoal with concomitant generation of steam.
Phosgene is generally produced on a technical scale by reacting chlorine with carbon monoxide in the presence of activated charcoal at a slightly elevated temperature (40.degree.-100.degree. C.). Since the presence of large quantities of chlorine would interfere with most uses of the phosgene, the chlorine content is kept as low as possible. In order to ensure a low chlorine content of the product phosgene, the carbon monoxide is used in an excess of 3-10% over the stoichiometrically required quantity. In addition, the considerable positive enthalpy of formation (108 kJ/mol) is removed by vigorous cooling because the phosgene synthesis does not proceed to completion at temperatures above 100.degree. C. (See e.g. Ullmanns Encyklopadie der technischen Chemie, 4th Edition, (1979), 18, pages 277-278).
On the basis of dissociation equilibria, phosgene contains about 50 ppm of chlorine at 100.degree. C. For many applications such a chlorine content constitutes the upper permissible limit. At 200.degree. C., as much as 0.4%, at 300.degree. C. 5% and at 400.degree. C. 20% of the phosgene is dissociated into carbon monoxide and chlorine. Consequently, all known processes are carried out in tubular reactors filled with activated charcoal from which reactors the heat of reaction may be rapidly removed by direct cooling with water, indirect cooling with inert cooling agents or evaporation cooling so that the temperature of the reaction gases leaving the reactor does not significantly exceed 100.degree. C. Careful cooling of the reaction tubes is also very important for reasons of operational safety since ordinary structural steel used for the reactors reacts vigorously at temperatures above 170.degree. C. to form iron chloride.
Under the usual operating conditions, virtually any good quality activated charcoal with a high absorption capacity is suitable for use as catalyst. The cooling medium temperatures after passage through the reactor are substantially below 100.degree. C. (generally at 50.degree.-70.degree. C.). Consequently, 1.09.times.10.sup.9 Joule per ton of phosgene cannot be used satisfactorily.
EP-Al-00 03 530 broadly discloses that the reactor used for the production of phosgene from chlorine and carbon monoxide may be cooled with boiling water to produce steam. No details are given. However, the production of high pressure steam is obviously not possible by this disclosed process because it is explicitly stated that the reaction product must be vigorously cooled immediately after the formation of phosgene (i.e. while still inside the phosgene reactor) to ensure that cleavage of the phosgene formed will not occur to any significant extent. This means that the temperature of the cooling medium cannot be substantially higher than 100.degree. C., and the production of high pressure steam is therefore not possible.